1. Field of the Invention
The present invention relates generally to hygrometers and, more specifically, to a fiber optic apparatus and method for sensing the dew point of an atmosphere in a process environment.
2. Discussion of the Related Art
Many methods of sensing the humidity levels present in a gaseous atmosphere are based upon inferring the moisture content of a sample from an output of a moisture-sensitive transducer. Such methods may be subject to such inherent limitations as a non-linear variation of the output parameter with humidity, drift, temperature sensitivity, hysteresis and aging. The limitation of temperature relates to the fact that the water-holding capacity of a gas varies with temperature so that only a relative humidity rather than an absolute measurement is provided. Relative humidity can be related to absolute humidity by using known conversion algorithms, such as the Goff-Gratch equation. However, this algorithmic solution can not be conveniently embodied in a linear circuit.
One method presently known and used extensively in industry for providing absolute humidity readings is to use chilled mirror hygrometers that provide an indication of dew point or frost point, each of which is a primary measurement of moisture content. The dew point is the temperature at which the partial pressure of a condensate on a surface equals the water vapor partial pressure in a gas. Similarly, frost point is the saturation temperature to which the gas temperature must be cooled at constant pressure so that it will be saturated in respect to ice. Saturation vapor pressure is a unique function of temperature. Therefore, determining the temperature at which water vapor begins to condense on a cool surface is equivalent to a measurement of its partial pressure.
Chilled mirror hygrometers use cooled mirrors as the surface where condensation takes place. Using electro-optic circuitry, a precise determination of the formation of either dew or frost is made. A temperature transducer, such as a platinum resistance transducer, is used to provide the signal indicating the output information.
One of the limitations of using such chilled mirror hygrometers in industrial applications is the inability of the electro-optic elements to differentiate between dew or frost deposits and dirt or soot deposits that may coat the mirror by particulate by-products of the process being measured. Since dew, as well as contaminants such as dirt deposits, will be measured by the included photosensors as a loss of reflected light, a differentiation between the dew and contaminant conditions must be made to ensure the accuracy of the hygrometer. Prior art responses in dealing with this type of contamination included cleaning procedures that are performed periodically or on an event-dependent basis. Another method employs compensation procedures in the hygrometers operation that "balance" the system. This is done by, in essence, performing a calibration operation to "null" variations in the optic sensitivity scheme due to the contaminant deposited on the mirror. This is accomplished by raising the temperature of the mirror to a temperature higher than the dew point to ensure a dry mirror so that the loss in reflectivity due to the contaminant alone may be measured. Adjustments are than made to the optical sensing circuitry to compensate for any differences in respect to the original calibration values.
Both these methods require taking the hygrometer "off-line", thereby losing the ability to monitor the process while the cleaning or compensation procedures are being performed. Additionally, as the contaminant deposits coating the mirror become more expansive, the compensation procedure explained above requires it be performed more frequently, increasing the time and frequency which the hygrometer must be off-line.